Relaxation dynamics in fluids of platelike colloidal particles

The relaxation dynamics of a model fluid of platelike colloidal particles is investigated by means of a phenomenological dynamic density functional theory. The model fluid approximates the particles within the Zwanzig model of restricted orientations. The driving force for time dependence is expressed completely by gradients of the local chemical potential, which in turn is derived from a density functional-hydrodynamic interactions are not taken into account. These approximations are expected to lead to qualitatively reliable results for low densities like those within the isotropic-nematic two-phase region. The formalism is applied to model an initially spatially homogeneous stable or metastable isotropic fluid which is perturbed by switching a two-dimensional array of Gaussian laser beams. Switching on the laser beams leads to an accumulation of colloidal particles in the beam centers. If the initial chemical potential and the laser power are large enough, a preferred orientation of particles occurs, breaking the symmetry of the laser potential. After switching off the laser beams again, the system can follow different relaxation paths: It either relaxes back to the homogeneous isotropic state or it forms an approximately elliptical high-density core which is elongated perpendicular to the dominating orientation in order to minimize the surface free energy. For large supersaturations of the initial isotropic fluid, the high-density cores of neighboring laser beams of the two-dimensional array merge into complex superstructures.